Re: What's in a Kinder harp mod?

[The Bernadettes <mdestefano@xxxxxxxx>]

Jim,
    This is mostly speculation on my part from what I could gather by the Kinder
website and from a conversation with someone who's been inside a Harp-King amp.
First, in the conversation the only thing I remember that stuck out was the
mention of "extremely low voltages."  I don't know if this meant pre-amp voltages
only or the entire amp meaning low B+ through out the power supply.  Also on the
Kinder site, it's mentioned about a low-mid cut as being part of the mod.  In
some of my experimenting in the past, I've rigged up several different amp and
mic combinations with a spectrum analyzer (the kind used by pro sound techs to
determine feedback frequencies.)  In my observations I found that the first
"ring" usually occurs somewhere in the 900hz to 3000hz region (depending on room,
mic, etc.), this is the high pitched upper midrange squeal the causes people to
stick fingers in their ears.  Once this is EQ'd out the second ring usually
occurs around 200hz, this is that "woofy" lower midrange feedback you get if you
crank up your amp with the treble and mids turned down.  I would presume the
low-mid cut part of the mod is some kind of frequency filter (basically the same
thing as a graphic EQ slider) wired into the signal path to filter out this low
mid feedback.  It's accomplished by wiring an inductor/cap/resistor circuit of
the appropriate values to deal with this offending frequency, look at some
graphic EQ schematics to get details on how it's done.  Unlike a graphic EQ, I
think it's only dealing with one filter instead of several.  The upper mid squeal
can be dealt with by both lowering the pre-amp voltages and changing some of the
values of the standard Fender tone stack to move the midrange dip higher to cover
that 900hz to 3000hz range.  That's part of the tone stack mod on the Princeton
Reverb I shared with you.  Again the comments concerning specifics of the Kinder
harp mod is speculation on my part, and I would be interested in knowing if
anybody knows for sure.

Regards,
Don D.

James D Rossen and Dina Janzen wrote:

> Anyone tell what circuit changes comprise the Kinder harp mod on a Bassman or
> Concert?
>
> Jim R
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